Ice could not have formed the blockages in Lochaber unless in every case the water escaped over some col into a contiguous valley on the same watershed, or into the eastern watershed. Supposes that the cols were not land-straits, but the places where the lakes were drained when forced to flow the wrong way.

Transcription

London

22 Oct. | 1861

My dear Darwin

The Swiss engineers said that they tunneled the ice barrier in the case of the valley
of Bagnes because if once the water overflowed it would cause a debacle.

Therefore I hold that ice could not have formed the blockages in Lochaber unless we can
assume that in every case the water escaped over some `col' either into a contiguous
valley on the same watershed, or over into the eastern watershed—

Whether the barrier be of ice or moraine-matter it cannot stand long eno' at one level
to allow a shelf to form if the water issues at the ice-dam or moraine-dam— On
the other hand if you suppose a continual increase of ice or moraine at the lower end of
each glacier-lake you would have an ever-varying depth of water.

There is only one way to get a permanent level & that is by the water
over-flowing some ``col'' distinct from the lower or dammed-up extremity of the lake.

If you can cite any case in Lochaber where a shelf does not coincide with a ``col''
(did not Milne or some one say there was one?) I cannot
comprehend how it is to be reconciled with the ice or glacier-lake theory— I
therefore do not suppose the ``cols'' were land-straits but the places where the lakes were drained when forced to flow the wrong
way—

Please to return this note that I may send it to Mr Jamieson &
see if he agrees—

The Val de Bagnes runs along the river Drance to the south-east of Martigny in the
canton of Valais, Switzerland. Lyell refers to the ice and snow that commonly fell from
the mountainsides and accumulated, impeding the flow of the Drance near Mauvoisin. In
June 1818, after an exceptionally large accumulation of ice the previous
winter, the lake that was formed above Mauvoisin burst through the ice barrier, causing
devastation to the entire Val de Bagnes (Baedeker 1889, p. 292).

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For many years, the cause of the famous parallel terraces or `roads' of Glen Roy in
the district of Lochaber, southern Inverness-shire, Scotland, had been debated. For CD's
theory of the marine origin of the parallel roads, published in 1839, see `Parallel
roads of Glen Roy'; see also Rudwick 1974. CD corresponded on the subject with
Lyell and Thomas Francis Jamieson, after Jamieson's visit to Lochaber in
August 1861 (see Correspondence vol. 9, Appendix IX).
Jamieson became convinced that the roads represented the shores of former glacial lakes
and that the level of these shores coincided with the level of various `cols' or passes
at the head of valleys (glens), indicating that the cols had once been points of
outflow. Crucial to his hypothesis was the proposal that glacial barriers had at one
time existed at the mouths of the glens. In a letter of
14 October 1861, Jamieson suggested to Lyell that the barriers of the
Glen Roy lakes were `mainly of ice' (Correspondence vol. 9, Appendix
IX). After reading Jamieson's letter, CD had expressed doubts about the possibility of a
`mere barrier of ice' being able to account for the formation of shelves at Glen Roy
(see Correspondence vol. 9, letter to Charles Lyell,
20 October [1861] and n. 1). Lyell, by analogy with the case of the
Swiss ice barrier, evidently concluded that ice barriers in Lochaber would have given
way under the force of the water or, if the ice were continually increasing, have
resulted in a constantly changing depth of water. This argument was based on the
supposition that the ice barrier would have been at a lower level than the col so that
water would overflow the barrier rather than the col.

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David Milne had found a col at Glen Glaster, at the same level as one of the
shelves, which had been missed by previous observers. He also mentioned two intermediate
shelves, faintly discernable, which he took as evidence of the gradual lowering of a
barrier (Milne 1847, pp. 398, 404).

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Land-strait: `isthmus' (OED).

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Lyell evidently sent this letter to Jamieson, who, in his reply, agreed with Lyell's
conclusion that cols had determined the location of the shelves and that water had
overflowed the col rather than the barrier. He dismissed the conflicting evidence of
intermediate shelves described by Milne, noting how faint these markings were (see
Correspondence vol. 9, Appendix IX, letter from
T. F. Jamieson to Charles Lyell, 27 October 1861).